This invention relates to a process for ensuring a neutral vehicle handling during cornering and a simultaneous load change and, more particularly, to a process for ensuring a neutral vehicle handling during cornering and a simultaneous load change by the operation of a vehicle system having at least one driven axle, an axle differential gear assigned to the driven axle, wheel brakes for the selective deceleration of an individual wheel, a device for recognizing cornering, and a device for recognizing a coasting operation and for generating a signal corresponding to the intensity of the coasting operation.
It is known to use mechanical limited-slip differentials for stabilizing the vehicle handling of motor vehicles. In this case, for example, the traction in the drive operation on roads having a low coefficient of friction is optimized and a clear improvement of the load change behavior is achieved specifically during cornering. However, limited slip differentials have the disadvantage that a basic locking moment and the load-dependent proportion of the locking effect are firmly predetermined. If, as the result of a high engine drag moment, the friction stress on the driven or dragged axle is exceeded, the mechanical lock has a negative effect. In addition, in the case of mechanical limited slip differentials, an antilock operation can no longer take place in the desired manner.
The use of electronic-hydraulic wheel slip control systems is known. Conventionally, they are used to cause a controlled braking intervention in the starting stage so that a limited slip differential effect can be generated.
The implementation of differential locks by an electric or electro-hydraulic braking intervention is described in German Patent document DE 33 30 236 C2, as well as in German Patent document DE 41 32 470 A1 (corresponding to U.S. Pat. No. 5,397,174), among others.
However, by means of the above-mentioned systems, no neutral vehicle handling during cornering and a simultaneous load change particularly in the coasting operation - can be carried out. In these driving conditions, a braking of the driven wheels takes place as the result of the engine braking effect. This results in a more or less large negative slip of the driving wheels. Depending on the driving method (rear-wheel drive or front-wheel drive) and the corresponding coasting force, the vehicle therefore has the tendency to oversteer or understeer. This tendency is schematically illustrated in FIG. 1. The center of gravity of a vehicle with a neutral cornering behavior moves about a center M with the radius R on the solid line which is marked "neutral vehicle". Rear-wheel driven vehicles tend to oversteer so that the cornering radius is reduced. This is characterized in FIG. 1 by the broken line marked "oversteering vehicle". Correspondingly, in the case of a front-wheel driven vehicle, there is the problem of an understeering so that, in comparison to the neutral vehicle, there is a larger cornering radius, as illustrated in FIG. 1 by the line marked "understeering vehicle".
There is therefore needed a process which ensures a neutral vehicle handling and a simultaneous load change.
This need is met according to the present invention by a process for ensuring a neutral vehicle handling during cornering and a simultaneous load change by the operation of a vehicle system having at least one driven axle, an axle differential gear assigned to the driven axle, wheel brakes for the selective a deceleration of an individual wheel, a device for recognizing cornering, and a device for recognizing a coasting operation and for generating a signal corresponding to the intensity of the coasting operation. A wheel of the driven axle during cornering is braked at least as a function of the coasting operation signal such that the moment generated thereby (counter-yawing moment) compensates the yawing moment caused by the cornering during the coasting operation.
According to the present invention, it is essential that, for improving the load change behavior during cornering, a counter-yawing moment is generated on the dragged axle via a controlled braking intervention. The counter-yawing moment counteracts the yawing moment caused by the coasting operation. By means of an optimal compensation of the yawing moment occurring during cornering and a simultaneous coasting operation, a neutral vehicle handling is achieved. The operator can therefore complete the cornering without having to expect an oversteering or understeering of the vehicle.
As the result of the development of forces in the case of rear-wheel drive and/or front-wheel drive vehicles, the front wheel or rear wheel which is on the outside or on the inside during the cornering is braked depending on the understeering or oversteering tendency. Because of the braking of the corresponding wheel, the already existing drag forces caused by the engine braking are increased to a resulting force. By contrast, because of the effect of the axle differential on the other wheel of the axle, the drag force is correspondingly reduced. Because of the resulting braking effect differences, a moment is generated on the whole whose direction is precisely the opposite to the yawing moment caused by the cornering and the coasting operation. A complete compensation will take place when the amount of the counter-yawing moment corresponds precisely to the amount of the yawing moment.
For implementing the process according to the invention, on the one hand, a cornering of the vehicle must be recognized and, on the other hand, the direction of the cornering must be recognized. This can be achieved, for example, by the determination of the differential speed of the wheels on the nonpowered axle and/or of the steering angle and/or of the lateral acceleration.
In order to obtain an optimal compensation of the yawing moment, the extent of the coasting operation should also be determined. This can take place by a comparison of the torque desired by the driver with the frictional moment of the engine, or by the comparison of the throttle valve setting by the driver with the characteristic zero moment curve of the throttle valve in the characteristic engine moment diagram. If the coasting force caused by the engine is known, a conclusion can be drawn on the resulting yawing moment.
A different vehicle handling occurs on roads with a different coefficient of friction. For this reason, it is very advantageous for the coefficient of friction of the road to be included in the calculation of the braking pressure for the brake of an individual wheel which is to be generated for a specific counter-yawing moment. The coefficient of friction of the road can be determined, for example, from the different wheel speeds and the vehicle acceleration.
Since the transmission ratio also plays a role in the determination of the drag forces or of the moments, it is advantageous to also take the engaged gear position into account for the determination of the braking pressure.
An advantageous embodiment of the process is characterized in that the braking operation for generating the counter-yawing moment takes place starting from specific drag forces or wheel slip values. For this purpose, a fixed quantity is determined, for example, a quantity corresponding to the engine drag moment or the wheel slip. The process according to the invention will not be implemented until this quantity exceeds a first limit value. Since the understeering or oversteering behavior can depend specifically on the coefficients of friction of the road, it is advantageous to determine this first control limit value as a function of the coefficient of friction of the road. Preferably., this control limit value is formed in the manner of a hysteresis in that the braking operation starts when a first hysteresis curve is exceeded and is terminated in the case of a falling below a second hysteresis curve. This prevents oscillation in the control circuit. The difference between the two characteristic hysteresis curves can be selected, for example, at about 0.25 km/h.
A particularly advantageous embodiment of the process is characterized in that the first control limit value is determined by a deviation of the speed of the wheel to be braked from the actual vehicle speed. As an alternative, the wheel slip of the wheel to be braked may also be taken into account. These embodiments of the process can be implemented particularly easily because the wheel speeds can be determined without any additional expenditures by means of rotational wheel speed sensors normally existing on the vehicle.
It may be a problem that, as a result of the additional braking of a driving wheel, there is the danger of a cessation of the lateral control of this wheel This is true particularly in the case of rear-wheel driven vehicles when the rear wheel which is on-the outside during the cornering is braked. If the braking force is too high, the overall force resulting from the drag force and the additional braking force reaches such an extent that there is a danger that the lateral control of the vehicle may be lost. This must be prevented. For this reason, the process according to the invention should be terminated in order to ensure continuous lateral control. This is achieved, for example, in that the process according to the invention is terminated when a second control limit value is exceeded. This second control limit value is farther away from the actual speed of the vehicle than the first control limit value.
The process according to the invention will preferably not be terminated before the second control limit value has been exceeded for a specific time period. It is also an advantage for the road friction to be included in the calculation of this time period.
When the process according to the invention is carried out, because of the additional braking of a driven wheel, on the whole, a negative vehicle acceleration occurs which is higher than the deceleration desired by the operator by releasing the accelerator pedal. This additional braking effect can be compensated by a corresponding increase of the engine torque. Since some vehicles already contain an engine drag moment control, this control can be used for compensating the additional negative acceleration. The increase of the engine torque is a function of the additional brake pressure and vehicle-specific data. The increase of the engine torque may therefore also be calculated from these parameters.
A decisive quantity for the counter-yawing moment or the increase of the engine torque is the brake pressure used for the additional braking. The brake pressure must be determinable in any driving condition. A determination possibility is obtained by the addition of the pressure build-up pulses and the subtraction of the pressure reduction pulses in the case of a known P-V diagram.
In addition, the process according to the invention should not be used during normal braking or during the antilock operation of the brakes of the vehicle so that, at a corresponding signal, the process according to the invention should not be carried out or should be terminated.
In an advantageous embodiment, the process according to the invention is also not carried out at speeds less than a specific absolute limit speed, for example, 20 km/h. This leads to a higher vehicle safety and to the avoidance of a pushing operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.